Energy desaturation of electromechanical actuators used in satellite attitude control

Abstract

In an attitude control system for a spacecraft having a frame which is to be maintained in a desired orientation relative to a selected set of reference axes, which system includes an electromechanical actuator mechanically coupled to the spacecraft frame for applying to the frame a mechanical torque which opposes deviations from the desired orientation relative to at least one reference coordinate, the actuator including an electric motor and an element connected to be rotated by the motor, and a torque command signal generator connected to the motor for applying a torque command signal which is a function of such deviations and which drives the motor in a manner to control the torque being produced by the actuator, there is included an energy desaturating system composed of: a unit coupled to the element for producing a signal representative of element motion; a signal modifying circuit connected to produce a control signal having a magnitude which is a function of the signal representative of element motion; and signal coupling components including a switch connected for applying the control signal to the actuator with a polarity for effecting actuator desaturation only during the times that the switch is closed.

What is claimed is: 
     
       1. In an attitude control system for a spacecraft having a frame which is to be maintained in a desired orientation relative to a selected set of reference axes, which system includes an electromechanical actuator mechanically coupled to the spacecraft frame for applying to the frame a mechanical torque which opposes deviations from the desired orientation relative to at least one reference coordinate, the actuator including an electric motor and an element connected to be rotated by the motor, and torque command signal generating means connected to the motor for applying a torque command signal which is a function of such deviations and which drives the motor in a manner to control the torque being produced by the actuator, the improvement comprising energy desaturating means comprising: means coupled to said element for producing a signal representative of element motion; signal modifying means connected to produce a control signal having a magnitude which is a function of the signal representative of element motion, said signal modifying means comprising a signal amplifying device having a fixed gain; and signal coupling means including a switch connected for applying the control signal to said actuator with a polarity for effecting actuator desaturation only during the times that said switch is closed. 
     
     
       2. A system as defined in claim 1 further comprising auxiliary torque means for applying to the frame an auxiliary torque which opposes deviations from the desired orientation when said switch is closed. 
     
     
       3. A system as defined in claim 2 wherein said auxiliary torque means is constructed such that, when said energy desaturating means is inactive and the spacecraft is in its desired attitude, the restoring torque generated by said auxiliary means is in equilibrium with disturbance torques acting on the spacecraft frame. 
     
     
       4. A system as defined in claim 1 wherein said actuator comprises a reaction wheel assembly having a wheel which constitutes said element. 
     
     
       5. A system as defined in claim 1 wherein said actuator comprises a momentum wheel assembly having a wheel which constitutes said element. 
     
     
       6. A system as defined in claim 1 wherein said actuator comprises a control moment gyro having a rotor and a pivotally mounted gimbal supporting said rotor, and said gimbal constitutes said element. 
     
     
       7. In an attitude control system for a spacecraft having a frame which is to be maintained in a desired orientation relative to a selected set of reference axes, which system includes an electromechanical actuator mechanically coupled to the spacecraft frame for applying to the frame a mechanical torque which opposes deviations from the desired orientation relative to at least one reference coordinate, the actuator including an electric motor and an element connected to be rotated by the motor, and torque command signal generating means connected to the motor for applying a torque command signal which is a function of such deviations and which drives the motor in a manner to control the torque being produced by the actuator, the improvement comprising energy desaturating means comprising: means coupled to said element for producing a signal representative of element motion; signal modifying means connected to produce a control signal having a magnitude which is a function of the signal representative of element motion; signal coupling means including a switch connected for applying the control signal to said actuator with a polarity for effecting actuator desaturation only during the times that said switch is closed; and auxiliary torque means for applying to the frame an auxiliary torque which opposes deviations from the desired orientation when said switch is closed, said auxiliary torque means comprising at least one active component for generating the auxiliary torque, auxiliary torque generating means for generating an auxiliary torque command signal which is a function of such deviations, and second signal coupling means including a second switch connected for applying the auxiliary torque command signal to said active component only during the times that said second switch is closed. 
     
     
       8. A system as defined in claim 7 wherein said auxiliary signal generating means comprises an amplifying device having a fixed gain. 
     
     
       9. A system as defined in claim 7 wherein said switch and second switch are operated in unison. 
     
     
       10. In an attitude control system for a spacecraft having a frame which is to be maintained in a desired orientation relative to a selected set of reference axes, which system includes an electromechanical actuator mechanically coupled to the spacecraft frame for applying to the frame a mechanical torque which opposes deviations from the desired orientation relative to at least one reference coordinate, the actuator including an electric motor and an element connected to be rotated by the motor, and torque command signal generating means connected to the motor for applying a torque command signal which is a function of such deviations and which drives the motor in a manner to control the torque being produced by the actuator, the improvement comprising energy desaturating means comprising: means coupled to said element for producing a signal representative of element motion; signal modifying means connected to produce a control signal having a magnitude which is a function of the signal representative of element motion, said signal modifying means comprising a circuit arrangement which produces an adjustable amplitude relation between the signal representative of element motion and the control signal; and signal coupling means including a switch connected for applying the control signal to said actuator with a polarity for effecting actuator desaturation only during the times that said switch is closed. 
     
     
       11. A system as defined in claim 10 wherein said circuit arrangement comprises means responsive to a signal originating from outside the spacecraft for adjusting the amplitude relation. 
     
     
       12. A system as defined in claim 11 wherein: said circuit arrangement is analog circuit; said means responsive to a signal comprises a component having a variable impedance; and said component is connected such that the adjustable amplitude relation is a function of the value of the impedance and the impedance is varied by the signal originating from outside the spacecraft. 
     
     
       13. A system as defined in claim 12 wherein said component comprises a potentiometer and said circuit arrangement further comprises a motor connected to adjust said potentiometer in response to the signal originating from outside the spacecraft. 
     
     
       14. A system as defined in claim 11 wherein: said circuit arrangement is a digital circuit comprising: a memory having a plurality of groups of memory locations, each group being associated with a respective amplitude relation and storing a succession of representations of control signal values corresponding to respective values of the signal representative of wheel motion; and signal decoder means connected to said memory for deriving the control signal from representations stored in a respective group of memory locations. 
     
     
       15. A system as defined in claim 14 wherein said circuit arrangement further comprises means connected to said memory for selecting the group of memory locations from which the control signal is derived in dependence on the value of the signal originating from outside the spacecraft.